This invention generally relates to display devices. More particularly, this invention relates to display devices having resolution control systems for one or more operating parameters.
Display devices are used in a variety of consumer and industrial products to display data, charts, graphs, messages, other images, information, and the like. Backlight display devices, which may be backlit or frontlit, have a backlight positioned to provide light for a display panel. Emissive display devices have pixels that are the emissive light source. In emissive displays, the pixel light source may be a CRT phosphor, a FED phosphor, a light emitting diode (LED), an organic LED, an electroluminescent, or any emissive display technology. In backlight display devices, the backlight may be a fluorescent tube, an electro-luminescent device, LED, a gaseous discharge lamp, a plasma panel, and the like. The display panel may be a passive or active matrix liquid crystal display (LCD). The backlight and display panel are connected to control circuitry, which is connected to a voltage supply. The display device may be separate or incorporated with other components, such as a dashboard in an automobile or other vehicle, a portable electronic device, and the like.
Many display devices control operating parameters in relation to user preferences and the environment of the display device. These operating parameters include the brightness, contrast, color, tint, and the like. Some parameters remain at an essentially fixed level for an extended time period. Other parameters change frequently because of changes in the environment, user preferences, and similar factors. The control circuitry may automatically adjust one or more parameters in relation to changing environmental conditions of the display device. A user may further adjust or manually set one or more parameters through a user interface such as a knob, switch, keypad, touch screen, remote device, or the, like.
Each operating parameter typically may have multiple adjustment steps for changing the operating level. The adjustment steps may be arranged in an adjustment sequence, having a linear progression from the lowest to the highest operating levels for the parameter. The number of adjustment steps may depend upon the dynamic range of the display device. A wide dynamic range generally needs more adjustment steps than a narrow dynamic rage. The dynamic range corresponds to the use of the display device. A narrow dynamic range may cover one or a small number of uses such as daylight use, nighttime use, or the like. A wide dynamic range may cover several uses such as daylight use, nighttime use, dusk-to-dawn use, and the like. The number of adjustment steps also may depend on the desired resolution of the parameter. More adjustment steps generally provide more resolution than less adjustment steps with sufficient resolution. Other factors may increase the number of adjustment steps.
Many display devices support a large number of adjustment steps for one or more operating parameters. Applications with wide dynamic ranges usually provide sufficient adjustment steps to cover the ranges of use. In automotive applications, a display device may be used in a multitude of ambient light conditions ranging from bright, sunny days to dark, xe2x80x9cmoonlessxe2x80x9d nights and the like. Other applications also may have wide dynamic ranges.
In addition, some operating parameters may require variable resolution control. Parameters like brightness require variable resolution because of how a human eye perceives changes in operating levels of the parameter. The human vision system perceives changes in brightness and like parameters non-linearly and logarithmically. A user perceives a brightness change from about 10 nits to about 12 nits as essentially equal to a brightness change from about 100 nits to about 120 nits. As the brightness level decreases, more brightness control resolution provides brightness step changes perceived as uniform by a user. Thus, a backlight or emissive display device needs more brightness resolution at lower brightness levels and less brightness resolution at higher brightness levels. This variable resolution requirement unnecessarily increases the number of quantized levels that must be made available from the control circuit if accomplished in a linear manner, such as with a single digital to analog converter.
To change or adjust the operating level of a parameter, the control circuitry receives an input signal indicating a user preference, an environmental condition, a combination, or the like. The control circuitry uses the input signal to select an operating value of the parameter. For brightness, the operating value is a luminance value. A digital-to-analog converter (DAC) may be used to convert the operating value into an analog control signal or an output voltage, such as a command brightness signal. The control circuitry provides the analog control signal to the backlight, the display panel, or both, depending on the parameter. The control circuitry may modify or further adjust the analog control signal and may combine the analog control signal with other inputs to operate the display device at the desired level for the parameter.
The control circuitry typically has a single DAC or PWM plus a filter to convert the digitized control signal into the analog control signal. For some operating parameters, a high resolution DAC may be used to provide sufficient adjustment resolution for the lower levels. For brightness and similar operating parameters, an even larger DAC is used to provide an output signal and to reduce errors from quantizing the operating value. A DAC for brightness control may have 12 bits of resolution for use in a dynamic range of about 0.5 nits through about 400 nits. The higher resolution DAC may increase the cost of the display device. While the digital data input into the DAC typically has a linear progression, the analog control signals from the DAC need to have constant ratio steps or an exponential progression for a user to perceive uniform adjustment steps.
This invention provides variable resolution control of an operating parameter for a display device or any other device that requires more resolution at lower control variable levels. The operating parameter may be brightness, contrast, and the like. Two or more digital-to-analog converters or similar devices convert data values or digitized control signals into analog control signals or output voltages for controlling the operating parameter. The two or more digital-to-analog converters have a cascade arrangement.
In one aspect, a display device has digital-to-analog converter (DAC) circuitry connected to a lighted display panel. The DAC circuitry has at least a first DAC and a second DAC. The first DAC is operatively connected to provide a first output voltage to the second DAC. The second DAC is operatively connected to provide a second output voltage to the lighted display. The second output voltage is responsive to the first output voltage.
In another aspect, a resolution control system has a first DAC and a second DAC. The first DAC has a first voltage output, a first data input value, and a reference voltage input. The second DAC is operatively connected to receive the first voltage output of the first DAC. The second DAC has a second voltage output, a second data input, and an input for the first voltage output.
In a method for controlling the resolution of an operating parameter, a first data input value and a second data input value are provided. The first data input is converted into a first output voltage as a function of a reference voltage. The second data input is converted into a second output voltage as a function of the first output voltage.
In another method for controlling the resolution of an operating parameter a control signal is generated with at least two cascaded digital-to-analog converters. A parameter of a display device is controlled in response to the control signal.
Other systems, methods, features, and advantages of the invention will be or will become apparent to one skilled in the art upon examination of the following figures and detailed description. All such additional systems, methods, features, and advantages are intended to be included within this description, within the scope of the invention, and protected by the accompanying claims.